The fluorescence properties of proteins and tryptophan derivatives were studied using conventional and laser-excitation spectroscopic methods. A discrepancy between the fluorescence decay kinetics and the fluorescence quantum yields indicated that these compounds exist in solution as equilibrated mixtures of conformers, some of which are essentially nonfluorescent. Both the tyrosyl and tryptophanyl groups of native proteins can lie in proximity to strong quenching groups which could cause static quenching. The ratio of lifetime/quantum yield is altered in denatured proteins where the amount of static quenching is often less. A basic fluorescence parameter known as the natural, or radiative, lifetime was successfully calculated for indole-containing compounds from their absorption and emission spectra. These calculations, in combination with fluorescence lifetime and quantum yield measurements, indicate that, as a rule, compounds containing both fluorescent and quenching groups in the same molecule are subject to static self-quenching. The protein alpha-lactalbumin undergoes a thermal transition near 52 degrees in the presence of Ca++, but at 35 degrees in its absence. The conformational change is marked by the appearance of fluorescence of a group which was essentially nonfluorescent at lower temperatures. This example of static quenching of protein fluorescence is being studied by time-resolved and steady-state fluorescence methods.
